aoa
Ross F. Lane and Arthur T. Hubbard
(3)E. M. Arnett in "Physico-Chemical Processes in Mixed Aqueous Solvents", F. Franks, Ed., American Elsevier, New York, N.Y., 1967,Chapter 6. (4)J. H. Stern and J. M. Nobilone, J. Phys. Chem., 72,3937 (1968). (5) H. S. Frank and F. Franks, J. Chem. Phys., 48,4746(1968). (6)E. G. Finer, F. Franks, and M. J. Tait, J. Am. Chem. SOC., 94, 4424 (1972). (7)R. L. Kay and T. L. Broadwater, flecfrochini. Acta, 16,667 (1971). (8)G.Wada and S.Umeda. Bull. Chem. SOC.Jpn., 35, 646,1797 (1962). (9)H. S.Frank and M. G. Evans, J. Chem. Phys., 13,507 (1945). (IO) R. K. Mohanty and J. C. Ahluwalia, J. Chem. Thermodyn., 4, 53 (1972). (11) J. C. Synnott and J. N. Butler, J. Phys. Chem., 73, 1470 (1969). (12)T. DeVries and H. Soffer, J. Phys. Chem., 55, 406 (1951). (13) W. T. Bolleter and R. J. Baczuk, Anal. Chem., 39,93 (1967). (14)P. W. Carr and J. Jordan, Anal. Chem., 44, 1278 (1972). (15) R. C. Weast, Ed., "Handbook of Chemistry and Physics", 51st ed, Chemical Rubber Go., Cleveland, Ohio, 1970-1971. (16)B. D. Faithful and S. C. Wallwork, Acta Crystallogr., Sect. 6,28, 2301 (1972). (17)P. G. Stecher, M. Windholz, and D. S. Leahy, Ed., "The Merck Index", Merck and Co., Inc., Rahway, N.J., 1968. (18) A. K. R. Unni, L. Elias, and H. I. Schiff, J. Phys. Chem., 67,1216 (1963). (19)B. C. Tyson, Jr., W. H. McCurdy, Jr., and C. E. Bricker, Anal. Chem., 33, 1640 (1961).
(20)J. J. Christensen, R. M. Izatt, and L. D. Hansen, Rev. Sci. hstrum., 36, 779 (1965). (21)"Philbrick Applications Manual", 2nd ed, G. A. Philbrick Researches, Inc., Dedham, Mass.. 1966,p 102. (22)P. W. Carr, Crit. Rev. Anal. Chem., IO, 491 (1972). (23)G.Choux and R. L. Benoit, J. Am. Chem. SOC., 91,6221 (1969). (24)R. G.Bates, J. Nectroanal. Chem., 29, l(1971). (25)R. L. Kay, G. P. Cunningham, and D. F. Evans in "Hydrogen-Bonded Solvent Systems", A. K. Covington and P. Jones, Ed., Taylor and Francis, London, 1968,pp 249-260. (26)A. S.Quist and W. L. Marshall, J. Phys. Chem., 72, 1536 (1968). (27)J. H. Stern and J. D. Kulluk, J. Phys. Chem., 73,2795 (1969). (28)T. S.Sarma and J. C. Ahluwalia, J. Phys. Chern., 76, 1366 (1972). (29)R. K. Mohanty and J. C. Ahluwalia, J. Solution Chem., I, 531 (1972). (30)R. K. Mohanty, S. Sundar. and J. C. Ahluwalia, J. Phys. Chem., 76,2577 (1972). (31)W. Y. Wen and J. H. Hung, J. Phys. Chem., 74, 170 (1970). (32)J. E. Desnoyers, M. Arel, G. Perron, and C. Jolicoeur, J. Phys. Chem., 73,3346 (1969). (33)F. Franks and D. S. Reid in "Water: A ComprehensiveTreatise", Vol. II, F. Franks, Ed., Plenum Press, New York, N.Y., 1973,pp 375-377. (34)V. E. Bower and R. A. Robinson, J. Phys. Chem., 67, 1524 (1963). (35)F. Franks, in ref 33,p 21. (36)Reference 33.D 350.
Electrochemistry of Chemisorbed Molecules. 111. Determination of the Oxidation State of Halides Chemisorbed on Platinum. Reactivity and Catalytic Properties of Adsorbed Species Ross F. Lane and Arthur T. Hubbard" Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 (Received July 26, 1974;RevisedManuscript Received December 6, 1974) Publication costs assisted by the National Science Foundationand the Petroleum Research Fund
Thin layer voltammetric data for Pt electrodes indicate that F-, C1-, Br-, and I- form chemisorbed layers which withstand rinsing with typical aqueous electrolytes. The chemisorbed halides are much less reactive toward electrochemical oxidation than the corresponding dissolved ions. Aqueous I- reacts with the Pt surface to form an uncharged adsorbed species, whereas F-, C1-, and Br'- are adsorbed in anionic form. Accordingly, F-, C1-, and Br-, but not I, induce the adsorption of Fez+ in a 2:l ratio of halide to iron. The influence of adsorbed halide on electrode reactions of metal ions such as Pt(II), Pt(IV), Sb(III), and Sb(V) can be understood simply in terms of electrostatic interaction between the reactant and the electrode surface. Chemisorbed F-, C1-, and Br- cause the surface to be less positively charged, whereas chemisorbed I has the opposite effect. In the irreversibly chemisorbed form, halide does not serve as an electron transfer bridge in the oxidation of Pt(I1) or Sb(II1).
Introduction A great variety of organic and inorganic ions and molecules chemisorb on Pt surfaces1,2 and when adsorbed profoundly influence the reactivity of electrode^^,^ and catal y s t ~ In . ~ particular, the halides and isoelectronic anions such as SCN- and S2- are strongly adsorbed, and when present appear to be intimately involved in electron transfer reactions, such as those of transition metal comp l e ~ e s . ~ ,Their " ~ involvement is of a fundamental sort and is expectable for all types of reactant^.^ Pt and Hg surfaces differ in some important respects in their behavior as adsorbents. For instance, adsorption of halides and olefins is considerably stronger on Pt than on Hg and the adsorbed layers are formed irreversibly; that is, they are not removed The Journai of Physical Chemistry, Vol. 79, No. 8, 1975
when the surface is subsequently rinsed with a solution of the pure s ~ l v e n t The . ~ ~ fact ~ that the charge of the adsorbed species does not necessarily correspond to its dissolved form, and thus the reactivity and influence may also be ~ n e x p e c t e d , ~combined ,~,~ with the fact that the charge and valency of species adsorbed a t Pt surfaces have not been determined prompted the studies reported here. The negative shift in the "rest" potential of a Pt electrode observed when KI is added to the aqueous electrolytic cell has been interpreted by Bagotsky et ~ 1 . asl ~due to specific adsorption of I-, in anionic form, and the correctness of their conclusions rests on the appropriateness of this assumption; there is considerable evidence to the contrary, however, that upon chemisorption the I- is convert-
809
Electrochemistry of Chemisorbed Molecules
(NaCEl
E (SECOND CYCLE)
- 20
B ’ ( S E C 0 N D CYCLE
1
B
Figure 1. Cyclic thin layer current-potential curves for clean and iodide-treated platinum electrodes: (- - - - -) clean electrodes; (-) electrodes containing chemisorbed iodine. The experimental conditions were as follows. The thin layer electrode was cleaned prior to each trial as described in the Experimental Section. Chemisorption of iodide occurred during exposure of the clean electrode to a 1 mF aqueous solution of KI for 3 min, at open circuit. The electrode was then rinsed thoroughly with the pure supporting electrolyte, 1 F HC104, to remove the excess, nonchemisorbed iodide from the solution layer; thus, only chemisorbed material was present at the onset of the current-potential curves. Thin layer volume, V = 3.88 pl; platinum electrode area, A = 1.15 cm2; average solution layer thickness, I = 33.7 y ; rate of potential sweep, 14 = 2.00 mV sec-’; solution temperature, T = 23 f lo. ed to a neutral s p e c i e ~ ~ 3 ~which 9 ~ J l displaces adsorbed anions and thus shifts the potential, &, at the outer Helmholtz plane to values more positive than would otherwise be observed. The observed shift in “rest” potential results, instead, simply from the fact that I- is an effective reducing agent. Enhancement of the electrooxidation of SbC14- in 2 F HC1 solutions at Pt electrodes resulting from pretreatment of the electrodes with KI has been interpreted as evidence that the chemisorbed species acts as an electron-transfer bridge between the electrode and the Sb(II1) reactant.12 Electrostatic double layer effects were excluded as a possible cause of the enhancement on the premise that I- adsorbs on Pt as the simple anion. However, the results could have been explained equally well by saying that adsorption increased 42, and in turn the effective reactant concentration, leading to the observed increase in rate in a manner identical with that already demonstrated for Pt complexe~.~>~J~ At issue, therefore, in the present work is the charge carried by species adsorbed from I- solutions on Pt, and the role played by the adsorbed species in electron transfer reactions. F-, C1-, and Br- have been investigated similarly to allow comparison. Results and Discussion 1. Interfacial Excess and Reactivity o f Chemisorbed Halides. i. Zodide. Thin layer current-potential curves for clean electrodes and ones containing a layer of chemisorbed
I appear in Figure 1. Oxidation of chemisorbed I commences a t potentials about 500 mV more positive than the standard potential of the 1211- couple in the initial, positivegoing potential scan (curve A), indicating that the chemisorbed material is hindered thermodynamically and/or kinetically from reacting. The oxidation products are nonchemisorbed IO3- 13 and adsorbed oxygen, as evidenced by peaks corresponding to reduction of 103- to I2 (curve B, peak potential, E , = 0.76 V NaCE) and the onset of reduction of the surface layer (curve C) in the subsequent negative-going scan. Following reduction of IO3- to I 2 a t the oxidized surface, concomitant reduction of 12 with the surface layer occurs (curve D), such that the layer of chemisorbed I is reestablished, as evidenced by the fact that subsequent scans are identical with the first one. That the chemisorbed oxygen layer formed during the initial, positive-going scan effectively prevents halide adsorption can be seen from the current-potential curve obtained when the scan direction is reversed following 103- reduction, curve B, but prior to oxygen-layer reduction (curves C and D), in which a new curve E appears corresponding in area and position to oxidation of dissolved 1 2 to IO3-, and from which curve A due to oxidation of chemisorbed material is absent. Rinsing the electrode with pure solvent just after the scan had passed curve B ( E = 0.70 V NaCE) completely removed the 1 2 from the cell, confirming that it was not present in an irreversibly chemisorbed state. Since current-potential curves for chemisorbed I obtained previously were not analyzed quantitatively to confirm their adherence to the particular form of the experimental equations dictated by the assumed mechanism,13 it seemed appropriate to do so here. The appropriate experimental equations were derived, tested,3 and reviewed,14 earlier. The thin layer current-potential curve for oxidation of an individual, chemisorbed halogenic species in an electrochemically irreversible process is expected to follow eq 1-3. Please refer to the Appendix for definitions of sym-
RT R T-2 y*,O* k) FoV (3 ) (1 - * a ) F In (1 bols employed. For the purpose of comparing experiment with theory, it is convenient to rewrite eq 1 in terms oE the peak current and peak potential, as follows: aE, = Eo f
Graphs of eq 4 and of the experimental data (corrected for contribution to the current due to background reactions as described in ref 3) appear in Figure 2. The agreement between experiment and theory is evidence for the correctness of the above interpretation. In plotting eq 4, it was assumed that the reaction involves a net transfer of five electrons ( n = 5 ; oxidation of zero-valent I to IO3-); direct determination of n was carried out to substantiate this assumption, as described below. The initial coverage was deThe Journal of Physical Chemistry, Vol. 79. No. 8 , 1975
810
Ross F. Lane and Arthur T. Hubbard
TABLE I: Thin Layer Coulometric Determination of the Number of Electrons Required for Oxidative Desorption of Chemisorbed la
Q - Qb,p C b (oxidation) 220.5 284.1 347.7 657.9 652.3 648.9
Q
-
Qb,
pCC
(reduction) 203.5 288.3 373.1 623.3 625 .O 642.4 1060.0 1123.6
109r,',mol cm-', from eq 6 0.37 0.52 0.67 1.128
1 .13d 1 .16d 1 .91e 2 .03e
-6
n, Faraday mol-', from eq 6 5.4 4.9 4.7 5.3 5.2 5 .O
a The experimental conditions were as follows. The supporting electrolyte was 1F HC10,; thin layer volume, V = 3.88~1;electrode area, A = 1.15 cm2; average solution layer thickness, 1 = 33.7 p. *The potential step was 0.400 t o 1.040 V NaCE. Values resulting from reduction of dissolved IOs-, immediately following the potential step b The potential step was 1.040 to 0.700 V NaCE. d Saturation value, obtained as in Figure 1. e These values resulted from rinsing the electrode with 1 mF IZ for 3 min, at open circuit. IZ solutions were prepared as described in ref 13.
4
Figure 3. Thin layer current-potential curves for clean and halidetreated Pt surfaces: (- - - - -) clean electrodes; (-) electrodes rinsed with aqueous sodium halide: (A) iodide, (8) bromide, (C) chloride, (D) fluoride. The experimental conditions were as follows. The electrode, cleaned as described in the Experimental Section, was exposed for 3 min at open circuit to 1 mFaqueous NaX (X = I, Br, GI, or F), followed by thorough rinsing with water and with pure supporting electrolyte. Current-potential curves for the clean surface were obtained by an exactly identical procedure in which pure H20 was substituted for the halide solution. The supporting electrolyte was 1 F H2S04. Other experimental conditions were as in Figure 1,
ed with chemisorbed I, essentially all of the I- introduced into the solution layer will become adsorbed, the interfacial excess resulting from filling the thin layer cavity with a total volume, V, of I- solution of initial concentration, Co, will be given by eq 5 . The quantity, n, in eq 4 is related to
ro=
(5)
CoV/A
the measured coulometric charge, corrected for a contribution from background reactions, by the Faraday law, eq 6.
Q -
Figure 2. Theoretical thin layer current-potential curve for electrooxidation of chemisorbed l (eq 4): (-. * - ) observed anodic current, ir, for electrode containing chemisorbed I; (-) theoretical current, it,,? for electrode containing chemisorbed I, graph of eq 4; ( a .)observed current for iodide-coated electrode, ir(corr), after correction for background current as explained in ref 3; (- - - -) background current, ic, for clean electrode; (- - - .) theoretical background current, ib, for a coated electrode. The following values were employed in preparing the plots: aip = 25.2 PA; = 1.07 V; = 1.12 X mol cm-2 (Table I); n = 5. Other parameters and experimental conditions were as in Figure 1.
-- -
-.
.
-
termined by means of thin layer coulometry, again based upon n = 5. P t surfaces containing analytically known quantities of chemisorbed I were prepared for use in determination of the n value for chemisorbed I oxidation by taking advantage of the fact that the adsorption process is quantitative and irreversible13 and that it is a simple matter by means of the thin layer electrode to expose the surface to a known volume of solution. Since, for a Pt surface not fully saturatThe Journal of Physical Chemistry, Voi. 79, No. 8, 1975
Qb
=z
(6 )
EFAr'
Equation 6 can be applied to the coulometric charge for reduction of 103- (n = 5 ) produced by oxidative desorption of I, as a test of the analytical accuracy of the above determination of n. The results appear in Table I, from which it is clear that oxidation of adsorbed I to 103- is a five-electron process. roapproaches a limiting value of about 1 X mol cmd2 as compared with approximately 2 X mol cm-2 of surface Pt atoms based upon geometric considerations, implying one adsorbed I per two Pt surface atoms. A possible explanation of this observation is that adsorption from I- solutions involves the formation of a mixed deposit of I and H, eq 7 . It is surprising that adI 1- + -Pt-Pt-
I
I
+
H,O
I
---t
-Pt-Pt-
I
H
t
t
+
OB-
(7)
sorbed H, if present, is not electrochemically oxidized at potentials as positive as 1.0 V NaCE, although adsorption of H on Pt, in a form functionally distinct from that derived from reduction of H20, upon chemisorption of hydrocarbons, has been reported.lS Adsorption of I from solutions of I2 was also studied. The results, Table I, indicate that when I2 is the surfactant, the saturation coverage ap-
81 1
Electrochemistry of Chemisorbed Molecules proaches one I per Pt in the surface layer, as would be expected if 1 2 is adsorbed without the necessity of sharing the surface equally with adsorbed H. However, the purpose of the present section is to describe the reactiuity of the chemisorbed halides; the charge of the adsorbed species has been investigated more directly through experiments described below. ii. Bromide, Chloride, and Fluoride. Current-potential curves obtained for clean Pt electrodes and ones exposed briefly to microliter volumes of dilute aqueous solutions of NaBr, NaCl, or NaF appear in Figure 3. Peaks related to reversible H deposition-dissolution appear in the currentpotential curves for clean electrodes and may be taken as evidence that the surface is relatively clean,15J6 although determination of the origins of these peaks evidently must await the results of experiments upon electrodes of precisely known surface atomic structure and composition. Similarly, positive-going scans exhibit several indistinct peaks in the oxygen-adsorption region which are associated with surface cleanline~s.1~ The influence of chemisorbed halide on the H and 0 adsorption regions of the current-potential curves is apparent for all of the halogens, but most dramatic for I. Exposing the Pt surface to 1 m F aqueous NaI completely prevents H and 0 electrodeposition over the normal ranges of potential characteristic of the clean surface. Similarly, treatment of the surface with aqueous Br-, C1-, or Fsuppresses H and 0 electrodeposition, although not as dramatically as I. The degree of influence is in the order I > Br- > C1- > F-. Halide adsorption has the effect of decreasing the amount of H deposited at a given magnitude of the electrode potential, as has been pointed out in many articles, listed in ref 16, and shifts the current peaks to less reversible potentials, as can be seen from Figure 3. Previous studies have not made clear the fact that chemisorption of each of the halides on Pt surfaces occurs irreversibly; that is, the chemisorbed halide is not removed by rinsing with inert electrolyte apart from extremely positive electrode potentials within a practicable length of time. Once the surface is exposed to halide it remains coated until cleaned. The characteristics of the Pt-halide bond in the chemisorbed layer are thus reminiscent of the nonlabile coordination compounds formed between halide ions and Pt(I1) or Pt(IV).ls Studies of the Pt-halide bond in adsorption situations and coordination compounds are in progress and will be reported elsewhere.lg Irreversibly chemisorbed halide layers on Pt have escaped detection a t least in part because the adsorbed material is difficultly reactive, giving no appreciable faradaic current in the range of electrode potentials in which the “dissolved” halides react f r e e l ~ . ~ ~Comparison ,*O of the current-potential curves in Figure 3, obtained when only chemisorbed halide was present, with those obtained with halide in the solution, Figure 4, illustrates this situation. I-, Br-, and C1- solutions all exhibit well-defined, reversible current-potential curves at the thin layer electrode, Fbeing masked by oxidation of the solvent, but the chemisorbed layers betray their presence only by suppressing the reactions of other species. Quantitative evidence has been presented for the unreactivity of I chemisorbed at Pt,’3 based upon experiments in which the clean thin layer electrode was supplied with only enough liquid to fill the thin layer cavity; due to the unreactivity of the chemisorbed material, the Faraday law takes the form of eq 8, resulting
8
-
Qb
= nFVCo - n F A r o
(8)
A -2
2 41-
-8r
A
A
Figure 4. Thin layer current-potential curves illustrating the unreacti-
vity of halide when chemisorbed on Pt: (curves A) electrode treated with large excess of halide solution; (curves B) electrode supplied with exactly 3.88 X low3cm3 of 0.5 mF NaX. The solution contained 0.5 mF NaX (X = I, Br) in 1 FH2S04.Other experimental conditions were as in Figure 1. Shaded area designates decrease in total charge caused by chemisorption in unreactive form.
in an area under the current peak (nFVCO - nFArO)of less than that expected under normal circumstances (nFVCO), achieved in the present case by rinsing the surface with an excess of the halide solution. The results appear in Figure 4. The difference in area between pairs of curves (shaded) mol cm-2 for the condiindicates that roequals about tions employed. Note that this is the maximum not the equilibrium value corresponding to the given concentration. However, owing to the large contribution to the measured charge resulting from background reactions, and the difficulty of properly assigning this charge, this procedure was not employed for quantitative estimation of P.Instead, after Br-, C1-, or F- had been allowed to adsorb, the coated surface was exposed to an exact volume of a solution of NaI in 1 F HC104, and the excess I- determined coulometrically, taking advantage of the circumstance that the adsorption process is irreversible, that I adsorption is quantitative and that adsorbed Br-, C1-, and F- do not interfere with the determination of I-. Typical current-potential data resulting from this procedure appear in Figure 5 . Each curve represents a separate experiment. Curve A, obtained in the absence of adsorbed Br-, indicates the amount of I- remaining after saturation of the initially clean surface with I (eq 8). Curve B shows that less I- is removed from solution by a surface pretreated with Br- and curve C shows that no further I- uptake is possible once the surface has been saturated with I. Curve D was obtained for an electrode containing only chemisorbed I. Curves having a similar appearance were obtained for surfaces treated with C1- or F-. In analyzing the resulting data it was assumed that chemisorbed I occupied all sites not occupied by Br-, in a ratio of two Pt surface atoms per I atom adsorbed and one Pt surface atom per Br- adsorbed (eq 9), where rIO(satd) and rIorefer to the maximum (satuThe Journal of Physical Chemistry, Vol. 75, No. 8, 1575
a12
Ross F. Lane and A r t h u r T. Hubbard
TABLE 11: Interfacial Excess Data for I, Br-, C1-, and F- Chemisorbed on Pta __
Halide
x
concn, m F
Q, PC
1.o 1.o 1 .o
274.0 i 2.3 350.3 i 6.6
23.9 i 1.0 28.1 1.2
317.4 i 4.2
24.0
I
Brc1-
iolorx-o, mol cm-2
F A r I o , PC,
Iodide
Qb,
from eq
PC
*
124.3 i. 3.3 52.2 i: 7.8
*
*
from eq 9
8
1.6
81.0
11.2 0.3 13.0 i 0.9 7.8 f 0.7 6.2 i 0.9
5.8
1.o 308.0 rt 7.1 23.5 i. 0.9 89.9 i. 8.0 a Chemisorption of Br-, C1-, and F- was effected as in Figure 3. Exactly 3.88 X 10-3 cm3 of 1.0 m F NaI in 1F HClOI was then added. Each entry represents the average of several trials with the reproducibility shown. Electrode area, A = 1.15 cm2; initial potential. 0.200 V NaCE; final potential, 0.550 V KaCE.
F'
TABLE 111: Interfacial Excess Data for Fez+ at Halide-Coated Pt Surfacesa
-2
ioiOrFez+O,
Halide
I
Halide concn, X mF
Q , IC
Qb,
mol cm-2, from eq 6
liC
-
* *
1.0 23.3 1.5 22.0 i- 1.1 0.1 i 0.2 Br1.0 98.4 f 2.7 31.8 i. 1.4 6.0 f 0.4 C1' 1.0 70.1 3.1 35.7 i 1.8 3.1 0.5 F1.0 69.2 k 3.0 41.4 i. 2.0 2.5 i. 0.5 a Experimental conditions as in Table 11, except C ~ ~ 2 -=0 0.5 mF; initial potential, 0.200 V NaCE; final potential, 0.520 V NaCF,. I
4t Figure 5. Thin layer current-potential curves for Pt electrodes containing mixed deposits of chemisorbed Br- and I: (curve A) clean cm3 of 0.5 mFNal, in 1 FHC104,added; electrode with 3.88 X (curve B) electrode was rinsed with 1 m F NaBr for 3.0 min; 3.88 X cm3 of 0.5 mFNal in 1 FHCIOl was then introduced;(curve C) electrode was rinsed with 0.5 mF Nal for 3.0 min; 3.88 X cm3 of 0.5 mF Nal in 1 F HC104 was then introduced; (curve D) electrode was rinsed with 0.5 mF Nal for 3.0 min; 3.88 X cm3 of 1 F HC104 was then introduced. Other experimental conditions were as in Figure 1.
rg,-@ = 2[rIo(satd)- rIo]
(9 1
ration) and observed (after Br- pretreatment) interfacial excesses, respectively. Equation 9 was also employed for C1- and F-. That I- does not dislodge chemisorbed Brfrom the surface is indicated by the absence of a peak for oxidation of dissolved Br- (Figure 5 , curve B). The results are summarized in Tables I1 and 111. 2. Adsorption of Fe2+ Induced by Halide. Kinetic evidence was presented in ref 4 and 6 that aqueous I- solutions react with Pt surfaces to form neutral chemisorbed species. Thermodynamic evidence in support of this hypothesis has now been obtained, also, by taking advantage of the fact that when a thin layer electrode is rinsed and filled with a solution of a species which adsorbs in an electroactive form the adsorbed amount is directly and accurately indicated by the coulometric ~ h a r g e , * , I ~eq, ~10. l In
Q-
Qb
+
= Y Z F V C ~ nFAro
(10)
particular, when a Pt thin layer electrode is rinsed with a solution of FeIz in HzO the surface acquires a layer of adsorbed halogenic species, and Fez+ ions will be retained a t the interface to preserve electroneutrality if the adsorbed halogenic species is negatively charged, but will be swept away into the bulk solution if the adsorbed halogen is uncharged; accordingly, observation of an interfacial excess of Fez+ would be indicative of halide adsorption in anionic The Journalof Physicai Chemistry. Vol. 79. No. 8. 1975
*
form, and, conversely, the absence of an interfacial excess would argue against the presence of anionic adsorbates. That is, the electrode electronic charge, q, must be equal and opposite to the sum of the ionic charges, qi,9 eq 11. Ne-
glecting the charge due to H+ and OH- for C F ~ Z2+ 10-4 F leads to eq 12. Thus, 2 1 ' ~ ~ 2 + and O rx-O differ by an amount q = -FALm[2C,,z+
- C,-]dx
=
-FA[2rF,z+O - TX-O] ( 1 2 ) equal to g, of course, but this difference is not sufficiently large to invalidate the qualitative result because the halide ionic charge excess, FAI'X-O, is very large in comparison with q, at least for X = I, Br, C1, and F. Thus, to preserve electroneutrality solely by equating g to FArX-0 would require unreasonably large electrode potential, capacitance, and/or zero charge potential. For instance, if rx-0 is taken to be mol cm-2, then the departure of electrode potential, E, from the zero charge potential, E,, predicted by means of eq 13 for electrode double layer capacitance, C, q = s,icclE
(13)
= FArX-O
on the order of 25 ~ Lcm-2,22,23 F would have to be about E E , = 4 V! Instead, it seems very likely that preservation of electroneutrality is formation of a mixed layer either as stated in eq 7 , or in eq 14. Fe(I1) exists primarily as the Fez+
-
Br' Br-
I
1
FeBr, + -Pt-Pt-Pt-Pt(14) I I I I simple aquo dication, Fe(HzO)G*+,in its dilute halide solutions at slightly acidic pH.Z4 Determination of Fez+ was carried out by means of thin layer coulometric experiments
ai3
Electrochemistry of Chemisorbed Molecules 1, pa -1.51
-1.51
1.51
1.51
D
t
n
-- I105 I
B
D
E
Figure 6. Current-potential curves of Pt thin layer electrodes rinsed with aqueous FeX2: (-) reactant solution; (- - -) background; (A) Fe12, (B) FeBr,, (C) FeCI2, (D) FeF,. The expe'rimental conditions were as follows. Prior to recording each current-potential curve the clean electrode was rinsed with an excess of 0.5 mF FeX2 in pure water at pH 6.5, freed of excess solution by application of pressurized inert gas and filled with exactly 3.88 X cm3 of 1 F HC104. NaX was employed in place of FeX, in recording the background curves. Other conditions were as in Figure 1.
--
in which it was ascertained that the excess of FeXz solution could be expelled from the thin layer cavity by application of pressurized inert gas, retaining only a thin film having a negligible volume, and that the cell could be subsequently filled with an aliquot of 1 F HC104 by release of the inert gas pressure without loss of the adsorbed material. Thus, the measured coulometric charge corresponded solely to the adsorbed Fez+ and background reactions (eq 6). Current-potential curves for Pt electrodes treated a t open circuit with aqueous FeI2, FeBr2, FeClz, or FeFz, freed of excess solution ( E = 0.200 V) and filled with 1F HC104 appear in Figure 6, and values of rpe2+0 are compiled in Table 111, from which it can be seen that halide ions induce the adsorption of Fez+ in the order Br- > C1- > F- and that FeIz does not lead to Fez+ adsorption, within the sensitivity of the method, a few per cent of a monolayer. This substantiates the claim that I- forms a neutral chemisorbed species on Pt. 3. T h e Question of Electron Transfer Bridging by Chemisorbed Halide. i. Electrochemical Interconversion of Pt (11)and Pt ( I V ) Compounds. Previous studies have detailed the influence of chemisorbed I on the electrode reac~ J ~comparison J~ tions of Pt(I1) and Pt(1V) c ~ m p l e x e s . ~ , By with various other surfactants, charged and neutral, it is apparent that chemisorbed I limits the adsorption of other anions, rendering the potential, 42, a t the reaction plane more positive than it would otherwise be, leading to an increase in the effective concentration of anionic reactants at the reaction plane and to an increase in reaction rate, or to a decrease in concentration and reaction rate of positively charged species, with little or no effect being observed for neutral reactant^.^ The kinetic influence of chemisorbed I and other surfactants not reacting chemically with the Pt(I1) and Pt(1V) complexes is preponderantly of electrostatic origin and depends only on the ionic charges of the reactant and surfactant for a wide variety of structural and
Figure 7 . Thin layer current-potential curves for Pt((NOz)(NH3))2 at clean and halide-coated platinum electrodes: (-) reactant sohtion; (- - - - -) blank: (A) clean electrode, (B) chemisorbed I, (C) chemisorbed Br-, (D) chemisorbed CI-, (E) chemisorbed F-, (F) solution contains l mF GI-. The solution initially contained l m F Pt(ll) and 1 F HC104. Other conditions were as in Figure 1. charge types. The remaining factors, steric hindrance and conformation, are important only at very high coverages or when reactant-surfactant chemical bonding3 is present. Further, the chemisorbed halide layer (the material not removed by rinsing and not electrochemically reactive a t the reversible potential) does not serve as an electron transfer bridge in Pt(I1)-Pt(1V) interconversion. That is, an electrode known to contain chemisorbed halide does not oxidize Pt(I1) in halide-free solution by way of the characteristic halide-bridged path4,6,7J1,25,26 eq 15-20 (where Xequil
Pt"L4
+ z + Pt"L4Z
(Pt electrode) + XPt"L4Z
+
(15)
equi 1
(Pt electrode)-X-
(16)
equil
(Pt electrode)-Xf (Pt electrode)-X-Pt"L,Z
(17)
slow
(Pt electrode)-X-Pt"L4Z
--L fast
(Pt e1ectrode)-X-Pt"'L4Z
+
e-
(18)
+
e-
(19)
fast
(Pt electrode)-X-Pt1I'L4Z
+ slow
(Pt electrode)-X-PtIVL,Z equ11
(Pt electrode)-X-PtIVL4Z
===
(Pt electrode)
+
trans-PtIVL,XZ
(20)
The Journal of Physical Chemistry, Vol. 79, No. 8, 1975
814
= C1-, Br-, or I-, L = X-, SCN-, CN-, NOa-, “3, NR3, or HzO, in any combination, and Z = L or X-), as illustrated by typical data in Figure 7. Thus, apparently, halide in its irreversibly chemisorbed state on Pt is not nearly as effective in promoting the electron-transfer process,z4 eq 17 and 18, as halide present in a reversibly adsorbed state, or as the “free” ion. Comparison of experimental activation energies with values calculated by means of an extendedHiickel molecular orbital method has indicated that the strength of the electronic interaction between the bridging halide and the Pt(I1) complex is of primary importance in determining the reaction rate.26 Since the bond between the Pt surface and the bridging halide is at least as strong for chemisorbed halide as for reversibly adsorbed or “free” halide, it must be a weakening of the interaction between the bridge and the Pt(I1) complex that accounts for the ineffectiveness of chemisorbed halide. Thus, these experiments must be viewed in the context of two distinct states of adsorbed halide, halide in the chemisorbed state being less capable of interacting with the Pt(I1) complex (eq 17) and/or of conducting electron density from Pt(I1) to the electrode within the transition state complex (eq 18). Indeed, from the standpoint of electronic structure one would not expect species having such different surface chemical characteristics to behave similarly toward electron transfer. ii. Oxidation of Sb(IZI) at P t Electrodes in Aqueous HC1 Solutions. By analogy with the behavior of Pt complexes, the reported facilitation by adsorbed I of the oxidation of Sb(II1) in 2 F HC1 at Pt electrodes is not necessarily due to electron transfer bridging by the chemisorbed I, as was suggested,12 but may instead be due to the fact that adsorbed I causes 42 to have more positive values than otherwise with a corresponding increase in the surface concentration of Sb(III), present primarily as the anion, SbC14-,27,28which would account for the observed increase in rate. In order to clarify this point the electrochemical oxidation of Sb(II1) was studied under conditions identical with those employed by the previous workers, except that thin layer electrodes were used. In addition, the reaction was carried out at electrodes coated with ionizable substituted olefins capable of substantially altering the potential $2 but incapable of acting as effective electron transfer bridge^.^ The results are illustrated in Figure 8. Chemisorbed I facilitates the oxidation of Sb(II1) (curves A and B) but the products are a mixture of species (curve B) and not simply SbCle-, as previously reported.12 Of greater significance, however, is the fact that allylamine, allylacetic acid, and acrylic acid form chemisorbed layers on the Pt surface which facilitate the oxidation of Sb(II1) in the same manner as chemisorbed I (curves C-E), yet the structures of these species preclude effective electron transfer through them and their influence on the electrode rates of Pt(I1) and Pt(1V) complexes of various structural and charge types is fully accounted for by their tendency to inhibit C1- adsorption and to block access of the reactant to the electrode. Since similar rates are observed a t similar coverage or $2 value, it is evident that nonspecific electrostatic effects account for the influence of all of the surfactants, including chemisorbed I, and no occasion remains for postulating bridging by chemisorbed I in this instance.
Experimental Section Electrode rate measurements by means of thin layer electrodes have been described in papers dealing with the and reviewed,14 electrochemistry of Pt The Journal of Physical Chemistry, Vol. 79, No. 8, 1975
Ross F. Lane and Arthur T. Hubbard I.
ua
I
lo
Figure 8. Current-potential curves for SbCI4- at clean and surfacreactant solutions; (- - tant-treated Pt thin layer electrodes: (-) - -) background; (A) clean electrode; other electrodes treated with (B) iodide; (C) allylamine; (D) allylacetic acid; (E) acrylic acid. The experimental conditions were as follows. The electrode was cleaned and surfactant pretreated as described in the Experimental Section. Reactant solutions initially contained 1 mF Sb203 and 2 F HCI. Surmol cm-2, were as follows: factant coverages, in units of curve A, none; B, 11.2; C, 9.1; D, 6.3;E, 6.0. Other experimental conditions were as in Figure 1. recently. The thin layer electrode design employed for this work, a platinum rod positioned concentrically within a close-fitting precision Pyrex glass capillary, was described in ref 25. Electrode surface pretreatment is crucial in studies such as this and is rendered particularly difficult by the fact that chemisorbed layers were made to form on the surface. Highly reproducible surfaces free of spurious properties and exhibiting the characteristics commonly attributed to the so-called “clean” Pt surface were obtained as follows: heating in an oxidizing gas-oxygen flame, with intermittent quenching in 11N HC104, was followed by cooling for a few seconds in air and insertion into the precision glass capillary. A cyclic current-potential curve, recorded at 2 mV sec-l in deaerated 1F HClO4 from 0.4 to 1.3 to -0.3 and finally to 0.4 V (NaCE), served both to remove the last traces of adsorbed organic materials and to confirm the purity of the surface. The resulting surface was undoubtedly complex; experiments with single crystal surfaces, cleaned, annealed, and characterized in ultra-high vacuum, will be reported elsewhere.29 Prior to each experimental trial, or sequence, the electrode was further pretreated by alternate potentiostatic oxidation (1.2 V) and reduction (0.0 V), followed by equilibration a t 0.4 V (NaCE) in 1F HClO4 for a
ais
Electrochemistry of Chemisorbed Molecules minimum of 120 sec, with thorough rinsing a t each potential. In order to obtain reproducible surface coverage in various trials with the same surfactants, the clean electrode was exposed for exactly 180 sec to an excess of a deaerated, freshly prepared solution of the surfactant in pure HzO. The electrode was then rinsed with H20 and with supporting electrolyte, after which the current-potential curve was Dbtained. All solutions were prepared from reagent grade or specially prepurified chemicals and triply distilled H20. The temperature was 24 f lo. Solutions were deaerated with nitrogen. Potentials were measured relative to a calomel halfcell, prepared with 1 F NaCl, and denoted NaCE.
coulometric charge required for the blank solution electrode electronic charge, C 4 ionic charge due to species j 4j gas constant, J mol-' O K - ' R rate of potential scan, dE/dt, V sec-' Y T temperature, "K perpendicular distance from the electrode x surface, cm volume of thin layer cavity, cm3 V ionic valence of species j 2, NaCE = calomel reference half -cell prepared with 1 F NaCl References a n d Notes Qb
N. A. Balashova and V. E. Kazarinov, Russ. Chem. Rev. . [Enql. - Trans/.], 34, 730 (1965). E. Gileadi, "Electrosorption", Plenum Press, New York, N.Y., 1967. R. F. Lane and A. T. Hubbard, J. fhys. Chem., 77, 1401 (1973). R. F. Lane and A. T. Hubbard, J. Phys. Chem., 77,1411 (1973). G. C. Bond, "Catalysis by Metals", Academic Press, New York, N.Y.,
Acknowledgments. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation (Grant No. GP19747) for support of this re3earch.
1962
ippendix. Notation electrode area, cm2 activity of species j adsorbed in the compact layer, mol cm-3 charge transfer coefficient, apparent value differential double layer capacitance, Farad initial concentration of species j , mol cm-3 concentration of species j electrode potential, V formal standard electrode potential potential at which anodic peak current oc curred 2.718
Faraday constant, C equiv" interfacial excess of species j , mol cm-' iaj/ri
anodic component of current, A anodic component of current, peak value apparent standard electrochemical rate con stant, cm sec-' number of electrons transferred per molecule coulometric charge required for the reactant solution, C
(20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
J.-RrCushing and A. T. Hubbard, J. Electroanal. Chem., 23, 183 (1969). W. R. Mason and R. C. Johnson, J. Electroanal. Chem., 14,345 (1967). A. T. Hubbard and F. C. Anson, Anal. Chem., 38, 1601 (1966). P. Delahay, "Double Layer and Electrode Kinetics", Interscience, New York, N.Y., 1965. V. S. Bagotsky, Yu B. Vassilyev, J. Weber, and J. N. Pirtskhalava, J. Electroanal. Chem., 27, 31 (1970). A. L. Y. Lau and A. T. Hubbard, J. Nectroanal. Chem., 33, 77 (1971). R. J. Davenport and D. C. Johnson, Anal. Chem., 45, 1755 (1973). A. T. Hubbard, R. A. Osteryoung, and F. C. Anson, Anal. Chem., 38, 692 (1966). A. T. Hubbard, Crit. Rev. Anal. Chem., 3, 201 (1973). B. E. Conway, B. MacDougall, and H. A. Kozlowska, J. Chem. SOC., Faraday Trans. 1, 68, 1566 (1972). M. Breiter, Nectrochim. Acta, 8, 925 (1963). H. A. Kozlowska, B. E. Conway, and W. B. A. Sharp, J. Electroanal. Chem., 43, 9 (1973). F. R. Hartley, "The Chemistry of Platinum and Palladium", Applied Science Publishers, London, 1973. M. A. Leban and A. T. Hubbard, lnorg. Chem., to be submitted for publication. D. C. Johnson and S. H. Bruckenstein, J. Nectrochem. Soc., 117, 460 (1970). A. T. Hubbard and F. C. Anson, J. Nectroanal. Chem., 9, 163 (1966). M. Breiter, Electrochim. Acta, 8, 925 (1963). M. A. V. Devanathan and K. Rarnakrishnaiah, Electrochim. Acta, 18, 259 (1973). J. Bjerrum, L. G. Sillen, and A. E. Martell, "Stability Constants of MetalIon Complexes", 2nd ed, Chemical Society, London, 1964. C. N. Lai and A. T. Hubbard, lnorg. Chem., 11, 2081 (1972). C. N. Lai and A. T. Hubbard, lnorg. Chem., 13, 1199 (1974). G. P. Haight, Jr.. J. Am. Chem. SOC., 75, 3848 (1953). M. V. Vojnovic and D. B. Sepa, J. Nectroanal. Chem.. 31, 413 (1971). R. M. lshikawa and A. T. Hubbard, J. Elecfroanal. Chem., to be subrnitted for publication.
The Journal of Physical Chemistry, Vol. 79, No. 8, 1975
